RAF duplicate aircraft codes

[ChemicalChaos]

I stumbled across this earlier, and I was quite confused:

http://www.f4phantoms.co.uk/74-squadron-tail-lette...

It appears multiple aircraft have the same tail marking identifier, all at once in the same squadron. I wondered if the duplicated ones were replacements for writeoffs, but it appears not as they were all scrapped at the same time (post Phantom phase-out).

Can anyone please explain why they have duplicate aircraft identifiers?

[ChemicalChaos]

whiteonyx said:

The F4-J were sadly withdrawn from service and replaced with FGR-2s,

So the duplicated aircraft are replacements, they just presumably kept the old versions around for spares?

Also, I thought the J replaced the FGR2, as the J spec ones were the imports from the US Navy that came on strength towards the end of the RAF's Phantom service period?